Hi there DieterA,
Based on your setup I would recommend having two seperate SSO instances. As it is test/acceptation versus production.
You want to be able to test things out without messing up your production environment.
Are you currently using default certificates for all components of vCenter Server? Before upgrading to vSphere 5.1+ I would recommend using signed certificates to prevent any issues that could arrise when using self-signed certificates. While upgrading my test environment to vSphere 5.5 I had some certificate issues and had to replace the self signed ones for signed certificates (which can be an internal CA ofcourse).
I don't have any experience with linked-mode so let me know how things went! =)
Cheers!
René